Objectives, Method, and Materials

​Objectives

make inferences about the effects of limiting factors on sea turtle populations; and

make recommendations to minimize the factors that might lead to the extinction of sea turtles.

Method

Students become sea turtles and limiting factors in a highly active simulation game.

Materials

40 to 60 feet of rope or string; two jump ropes or hula hoops; one paper or plastic bag per student; identity cards for each predator or limiting factor (can be drawn by students); wooden clothes pins; poker chips; dried beans

Background

Sea turtles are survivors of the great age of the dinosaurs and inhabit nearly all the oceans of the world. The best-known sea turtles are in the family Cheloniidae, which contains the green turtle, loggerhead turtle, hawksbill turtle, and ridley turtle. The huge leatherback turtle, the largest living turtle, is placed in Dermochelyidae, a separate family. Sea turtles live mostly in warm waters and have limbs modified into flippers. Female sea turtles crawl above the tide line to bury their eggs. Sea turtles leave the water only during these nesting periods. It is during this time that the turtles and their offspring are the most vulnerable to predation by humans and other wildlife.

As with most reptiles, turtles lay eggs. The eggs look somewhat like wet, pliable ping-pong balls. Using their rear flippers, female sea turtles dig deep holes on sandy beaches where they lay and bury their eggs. Mature female sea turtles may deposit several hundred eggs in one season. Once the eggs are buried, the females return to the sea or seek additional nesting sites.

A drawing depicting the life cycle of a sea turtle.The eggs incubate for nearly two months. If the eggs survive predation by raccoons, ghost crabs, foxes, dogs and humans, the sea turtles hatch, dig their way upward through the sand and promptly head toward the sea. Predatory crabs, raccoons and dogs, with gulls and other birds joining in, typically accompany the hatchlings' journey across the beach. Once hatched, only about one to five percent of the turtles survive the first year. In the sea, the turtles must mature for nearly a decade before returning to nesting sites as a natural part of their life cycle. Biologists are uncertain how long sea turtles reproduce and live. Here is an illustration of the life cycle of a sea turtle (right).

The motives for human predation are based predominantly on products that are outlawed in many countries. Jewelry, leather, oil, and food are the primary uses. Turtle eggs are seen by some as a boost to longevity and vigor; tens of thousands of eggs are illegally harvested for vanity sales. Evidence suggests that a serious human threat to the turtles is the poaching of their eggs in their nesting sites.

There are other, human caused factors. Dune buggies may break the eggs buried in the sand. More damaging, given the scope of the impact, is commercial and private construction (condominiums, private homes, hotels, etc.) on coastal sites. This may create a barricade that prevents the turtles from reaching their traditional nesting sites and eliminates many nest sites.

Entanglement in discarded fishing gear and plastic waste cast into the oceans is a serious hazard, killing many sea turtles each year. Many turtles fall accidental victim to the nets of large fishing trawlers. Once caught in the nets, they drown. Efforts are being made to popularize special trawling devices that will prevent turtles from getting into the nets. One of the turtles' favorite foods is jellyfish. Many turtles mistake the human-produced litter of floating plastic bags for this food. When eaten by the turtle, its digestive tracts become blocked with the' discarded plastic and they die.

Six of the seven known sea turtle species are officially designated either endangered or threatened. The leatherback, olive ridley, Kemp's ridley, hawksbill, green, and loggerhead are all listed as endangered species by the U.S. Fish and Wildlife Service.

Procedure

1. Set up the activity areas as shown in the diagram at the end of this p​age. Give each student a paper or plastic bag.

2. Divide the class into two groups.

Group 1—Turtles Each student counts out 50 beans to place in his or her bag. Beans represent turtles. Each bag of beans represents the turtles that hatch from a single nest.

Group 2 — Limiting Factors Divide this group into two smaller groups, on land and in-sea.

Give each student a sign that indicates what kind of limiting factor each one represents. Attach these identity signs to students' clothing with clothespins.

3. Walk the class through the activity and explain these rules:

A. Turtles must hatch, cross the beach and spend 10 years in the open sea. The turtles running between the year zones simulate the time in the ocean. They pick up one poker chip at a year zone and then run to the other year zone to pick up another poker ship. Each chip represents two years of successful ocean survival. After collecting five poker chips, turtles return to the nesting area to reproduce.

B. Turtles try to avoid limiting factors and predators. If tagged by a limiting factor, a turtle stops, counts out 10 beans and places those 10 beans in the limiting factor's bag.

C. The ocean's sea grass areas are turtle safety zones where limiting factors cannot tag them. The teacher may set a time limit for how long a turtle may rest in a sea grass zone. OPTIONAL: The educator may eliminate the safety zones after the turtles have been in the ocean for a while. This simulates the turtles growing too big to hide in the sea grass.

D. Limiting factors must obey the following rules:

They cannot tag the same turtle twice in a row.

They cannot tag turtles that are counting out beans to another limiting factor.

They must stay at least four steps away from any turtle that is transferring beans to another limiting factor.

E. Any turtle that loses all 50 beans is dead. It must go to the beach and become a condominium. If the condominiums (sitting side by side) eventually block the access to the nesting beach, the remaining turtles die without reproducing and starting the next cycle.

F. The activity ends when all turtles are either dead or have returned to the nest area.

4. Review the rules two times to make sure the students understand their roles and the procedures. Students then become endangered sea turtles and limiting factors and conduct the activity.

5. After completing the activity, encourage the students to discuss the results. It is likely that some students will be disturbed by the high mortality of the turtles and will benefit from the realization that there are groups actively trying to diminish human contributions to such high mortality. However, it is also important to emphasize that natural limiting factors are built into the scheme of things. If all sea turtle eggs survived, there might well be an overabundance of these creatures. Many animals produce more young than will survive, serving as food for other species as a part of nature's dynamic balance. Ask the students to briefly describe the life cycle of sea turtles.

6. Summarize the importance of the high numbers of turtles that result from the reproduction. Ask the students to identify and discuss the factors that limit the turtles' survival. Since sea turtles are threatened with extinction, the limiting factors affecting their survival seem to be out of balance. What specific recommendations would the students suggest to increase the successful reproduction and survival of sea turtles?

Extensions

​Change the ratio of predators and hazards to turtles (1/3 predators or hazards to 2/3 sea turtles) and replay the simulation. Describe and discuss the differences.

Set up a sea turtle information center.

Where possible, visit sea turtle restoration sites and determine what actions may be taken to enhance the stability of sea turtle populations.

Replay the activity with all human factors removed from influence.

Evaluation

Describe and illustrate the major stages of sea turtles' life cycle, beginning with the egg.

Name at least four limiting factors that prevent sea turtles from reaching the adult breeding stage.

Write a law that would help protect sea turtles. What would the law include? Who would enforce it?

​Diagram of layout for activity.

Nest Zone: the place where the eggs are laid and hatch (This is the zone to which the surviving turtles will return in ten years.) This is where the baby turtles hatch and begin their journey to the sea.)

Beach Zone: the zone the hatchlings must cross to get to the sea (It is a place of high predation and other limiting factor​s.)

Sea Zone: the area where the turtles must mature for a period of ten years before returning to nest.

Year Zones: the two zones that the turtles must visit to acquire the year cards necessary to "mature" to ten years of age (One poker chip is awarded for each one-way trip between the zones. During the trip between the zones, the turtles are vulnerable to predators and other limiting factors. Turtles are safe from other limiting factors when they are inside either year zone.)

Sea Grass Zones: places where the turtles are safe until they reach four years of age (At that age they are too large to hide from predators.)